A variety of systems for purifying exhaust gas have been commercialized for use as effective means for complying with emissions regulations. Among these, systems constructed having a catalyst encased in a cylinder so as to be disposed inside the exhaust pipe of a vehicle are in widespread use as systems for purifying vehicle exhaust gas. Fulfilling its purifying function causes the catalyst to become larger than the diameter of the exhaust pipe. Accordingly, it is necessary to reduce the diameter of an end part of the cylinder encasing the catalyst to match the front and rear diameters of the exhaust pipe.
The process for reducing the diameter is performed using spin-processing as disclosed in, e.g., JP 2002-239657 A. The spin-processing disclosed in JP 2002-239657 A will be discussed next referring to FIGS. 8A and 8B hereof.
As shown in FIG. 8A, a cylinder 101 is clamped by a lower clamp 102 and an upper clamp 103. Next, spinning rollers 104, 104, 104, 104 are caused to rotate while being pressed against one end 105 and another end 106 of the cylinder 101, whereby the diameters of the ends 105, 106 are reduced. This type of processing is called spin-processing or spinning.
When spin-processing is performed, the spinning rollers 104 are firmly pressed against the end 105 and the other end 106 of the cylinder 101; therefore, a large external force (bending force or torsion force) is applied to the cylinder 101.
The cylinder 101 is accordingly clamped securely between the lower clamp 102 and the upper clamp 103, as shown in FIG. 8B, so that the cylinder 101 will not move.
FIG. 9 shows a conventional clamp. In FIG. 8A the catalyst and the mat are not shown; however, in exhaust gas purification systems, a pillar-shaped catalyst 109 is inserted inside the cylinder 101 with a mat 108 interposed therebetween. As is clear from FIG. 8A, the catalyst 109 cannot be inserted in the cylinder 101 once the diameters of the ends have been reduced.
Therefore, as shown in FIG. 9, a semi-finished product 110 having the pillar-shaped catalyst 109 inserted inside the cylinder 101 with the mat 108 interposed therebetween is produced before spin-processing is performed. The semi-finished product 110 is then spin-processed; i.e., the semi-finished product 110 is held firmly between the lower clamp 102 and the upper clamp 103, whereupon spin-processing is performed.
A welded tube is generally used for the cylinder 101. Welded tubes are obtained by bending a flat plate into an “O”-shape and welding the butt seams together. Inevitably, irregularities arising from the dimensions of the flat plate, gaps in the butt seam, and contracting of the weld metal during welding combine to produce irregularities in the outside diameter of the cylinder 101.
In one known manufacturing method, a pillar-shaped catalyst is wrapped with a mat and inserted in a cylinder, and a size-reducing device is used to reduce the size of the cylinder, whereby the catalyst is accommodated therein. This method requires the diameter to be reduced to the proper degree since the catalyst will be inadequately secured if the diameter is insufficiently reduced, while the catalyst will be damaged it the diameter is excessively reduced. The fact that the diameter can only be reduced to a certain degree means that irregularities may arise in the outside diameter of the cylinder. Specifically, in the semi-finished product wherein the pillar-shaped catalyst wrapped in the mat is housed in the cylinder, the outside diameter of the cylinder may still be irregular even after the diameter of the cylinder has been reduced using a size-reduction device.
Problems associated with a conventional clamp will now be described with reference to FIGS. 10A and 10B.
As shown in FIG. 10A, a semicircular groove 112 corresponding to a reference outside diameter (average outside diameter) of the cylinder 113 is formed in the upper clamp 103. The outside diameter of the cylinder 113 will be larger than the reference outside diameter if the degree of irregularity is excessive. When the upper clamp 103 is clamped on the cylinder 113 having this large outside diameter, left and right corner parts 114, 114 of the semicircular groove 112 come into contact with the outer peripheral surface of the cylinder 113. Therefore, a gap d1 arises in the center of the semicircular groove 112.
In order to achieve a clamped state, it is necessary to lower the upper clamp 103 until the gap d1 disappears. When this is done, the left and right corner parts 114, 114 will cut into the cylinder 113, and the cylinder 113 will locally deform toward the center by an amount equal to the depth of the gap d1. The effect on the catalyst increases according to the extent of local deformation, and the catalyst may suffer damage. A recommended countermeasure addressing this concern is shown in FIG. 10B.
As shown in FIG. 10B, if the size of the upper clamp 103 is halved, a gap d2 will be dramatically smaller than the gap d1. The local deformation is ameliorated, and the effect on the catalyst is lessened.
Dividing the upper clamp 103 and the clamp into increasingly numerous segments thus progressively reduces the size of the gaps and lessens the effect on the catalyst.
However, the clamp 103 is subject to a reactive force from the cylinder 113. When the number of segments increases, the size of the clamp decreases, and the rigidity decreases, leading to concerns that the rigidity will be insufficient.
There is accordingly a demand for a chucking device that reduces the effect on the catalyst while preserving the rigidity of the clamps.